The field of digital image verification and protection, including verifying and protecting the integrity of digital images stored in memory for authentication and security purposes, has recently gained importance and wide recognition.
Several areas of research have been initiated in this field. One area of research focused on a "Trustworthy Digital Camera" in which a watermarking scheme was incorporated into an image to determine whether a digital photo had been modified, and the watermarking process included using existing public-key encryption technology. Another area of research studied coding undetectable digital signatures onto an image by the bit plane manipulation of least significant bit (LSB), as well as applying linear addition of the watermark to the image data using m-sequences. Other techniques hide simple data in images by embedding one bit of data in a host image using statistical approach (Patchwork), as well as using blocks of random textures to replace those regions of similar textures to create an identical pair of textured regions upon which the shape was recovered by auto-correlation measures (Texture Block Coding). Also, other research includes techniques to embed long strings of binary bits, called "digital seals" to a set of randomly generated addresses where the LSB of pixels were changed to match the corresponding bit in the string.
In many of the above techniques, the information is "stamped" into the LSB of the pixel values in the image. This stamping technique is unlikely to cause visual artifacts in the image. If an image is altered, the LSB is most likely to be changed such that the verification will be able to determine the modification. However, such LSB manipulation is not secure against malicious attacks: it is relatively easy to create a system to change the content of an image without changing the LSB of every pixel value, in fact, the whole image can be replaced, but as long as the LSB of every pixel is maintained as the original source image, the verification process is not able to detect such alterations. Another drawback of many existing techniques is that they are not able to determine the regions of modification in the verification process. The verification process will only be able to determine if the image is modified or not, but not able to locate where the alterations have taken place. Such information may be valuable for better security measures.
At one extreme, one stamping process is one in which the stamping process is null, and the information stamped into the image (the "stamping information") is the image itself. The verification process would then also be null and the extracted stamping information would be the stamped source image itself. The final comparison of the stamping information and the stamping information extracted would then effectively be a comparison of the source image known at stamping time with the source image known at verification time. This comparison is not very efficient, however, as the stamping information is quite large and requires considerable memory to store the information. Similarly, considerable bandwidth is required to transmit the information. This leads us to one desired property of the verification process is the amount of stamping information be quite small.
On the other hand, another stamping process is one that extracts a pixel from a predetermined location in the image and uses that pixel and its position as the stamping information. The verification process then determines whether this pixel was unchanged in the stamped image. While, in this case, the amount of stamping information is small, the certainty of correct verification is also low, since many different alterations of an image would not alter the single given pixel. Therefore, a second desired property of the verification process is that the verification process, with high likelihood, must be able to determine whether the stamped image has been modified since stamping.
In addition to image verification, digital watermarks have been proposed for copyright protection means. For example, in U.S. Pat. No. 5,530,759 entitled COLOR CORRECT DIGITAL WATERMARKING OF IMAGES, to G. W. Braudaway, K. A. Magerlein and F. C. Mintzer, a system for placing a visible "watermark" on an image is disclosed to discourage unauthorized use of the image. In U.S. Pat. No. 5,488,664, entitled METHOD AND APPARATUS FOR PROTECTING VISUAL INFORMATION WITH PRINTED CRYPTOGRAPHIC WATERMARKS, to A. Shamir, a method and device is disclosed for protecting visual information against unauthorized access and modification using a printed cryptographic "watermark" which is encoded using black and white pixels. These are examples of using "visible watermarks". The watermark itself is explicitly displayed and perceptually noticeable in the stamped image.